LibCpp: Replace defined preprocessor values when parsing

Author: https://github.com/itamar8910
Commit: 3658c4c567
Pull-request: https://github.com/SerenityOS/serenity/pull/5755

Base/home/anon/Source/little/other.cpp

@@ -5,6 +5,7 @@ int func()
 {
     int x = 1;
     int y = 2;
+    INT_Z = 3;
     StructInHeader mystruct;
     printf("x: %d\n", x);
     printf("y: %d\n", y);

Base/home/anon/Source/little/other.h

@@ -2,6 +2,8 @@ int func();
 
 #define USE_VAR2
 
+#define INT_Z int z
+
 struct StructInHeader {
     int var1;
 #ifdef USE_VAR2